1. Field of the Invention
The invention relates generally to the field of imaging devices and, more specifically, to control circuitry for digitally operated imaging devices.
2. Discussion of the Prior Art
Various methods and technologies exist for imaging printing plates. These include the use of electromagnetic-radiation pulses, produced by one or more laser or non-laser sources, to create chemical changes at selected points of sensitized plate blanks, which are used (immediately or after exposure to conventional development processes) for planographic printing; ink-jet equipment that is used to selectively deposit ink-repellent or ink-accepting spots on plate blanks, also to produce planographic printing plates; and spark-discharge equipment, in which an electrode in contact with or spaced close to a plate blank produces electrical sparks to alter the characteristics of certain areas on a printing surface, thereby producing "dots" which collectively form a desired image. As used herein, the term "imaging device" includes radiation sources, ink-jet sources, electrodes and other known means of producing image spots on blank printing plates, and the term "discharge" means the image-forming emissions produced by these devices. Multiple imaging devices may be used to produce several lines of image spots simultaneously, with a corresponding increase in imaging speed.
Regardless of the number of imaging devices used, the operation of the imaging devices must be precisely controlled so that the discharges occur at the appropriate times to reach the intended dot locations on the printing surface. If the operation of the imaging devices is not properly controlled, various undesirable characteristics may appear in the image. For example, in imaging systems which image printing plates mounted on rotatable cylinders, a condition which is referred to herein as "slanted swath" may be observed. The slanted swath condition is characterized by lines in the image which run in the axial direction as opposed to the circumferential direction, and which appear "sawtoothed" or jagged instead of straight.
The slanted swath condition may occur as a result of one or a combination of factors. First, in an imaging system which images a rotating cylindrical plate, a mechanism is required to monitor the rotation of the cylinder and provide angular position information for synchronizing the operation of the imaging devices. In order to accurately resolve the correct discharge locations, it is essential to generate precise position information. Such information may be provided by an angular-position encoder which "divides" the circumference of the cylinder into a predetermined number of increments and generates an appropriate output signal (e.g., a series of pulses, each of which represents a unit of distance around the circumference of the cylinder).
If multiple imaging devices are used for imaging, the circumferential distances between such devices must be precisely fixed to represent an integral number of units of circumferential distance. Otherwise, a "dimensional error" between the angular position information and the devices will exist, which will result in premature or delayed firing of the devices with respect to the rotating cylinder, which will in turn result in the slanted swath condition. Typically, normal manufacturing tolerances produce variations in the circumferential distances between devices which represent a significant dimensional error.
Manufacturing tolerances also produce variations in the dimensions (i.e., circumferences) of the printing plate cylinders. Thus, there is a likelihood that in a four-color imaging system which incorporates four separate cylinders (each which is paired with its own set of imaging devices) the four circumferences will not be the same. Accordingly, adjustments must be made to the operation of the imaging devices in order to produce four printing plates whose images are the same size in the circumferential direction. The most expedient way to make such adjustments is to alter the scaling or number of pulses produced by the angular position encoder. However, as described above, any change in the encoder's scaling will produce a dimensional error between the encoder and the imaging devices, which will again result in the slanted swath condition.
Another printing artifact that may occur in digitally imaged printing plates is a series of parallel lines that traverses the printed document along the direction in which the plate was imaged. These lines appear most prominently when the plate-imaging equipment includes multiple-device writing heads, and can arise from any number of causes (such as failure of individual devices to image at the same intensity as other devices, incorrect orientation of the writing head, or improper alignment of individual imaging devices within the head). For example, using a writing head consisting of a diagonal array of non-contact spark-discharge electrodes, we have found that the first electrode to make contact with the plate surface during each pass tends to produce image spots of diminished intensity; thus, streaks of uneven intensity will be produced even with a perfectly assembled writing head. Regardless of the source of the artifact, it will assert itself along each imaging pass, and its visual prominence will be augmented if the same cause affects, in register, all plates used to print an image. Assuming the source can be traced to a single errant imaging device or the stepping accuracy of the entire array, the frequency of the artifact will correspond to the width of the image strip produced by the writing head. Consequently, once an array of devices reaches a critical width, the artifacts it produces will be widely enough spaced to be visible to the eye, particularly where similar artifacts are printed in register.